As DRAMs increase in memory cell density, there is a continuous challenge to maintain sufficiently high storage capacitance within memory cells despite decreasing cell area. Additionally there is a continuing goal to further decrease cell area. Many methods have been proposed to keep the capacitance of such storage capacitors at acceptable levels. One approach is to increase the height of the storage node (electrode of the capacitor). Another approach is to use high dielectric materials such as Ta2O5, or BST.
However, there are some problems with the approach to increasing the height of the storage node. For example, if the required height of the storage node is more than 10,000 Å, it becomes very difficult to pattern conductive layers as storage nodes. There are also some problems with using high dielectric materials, such as Ta2O5 and BST, as dielectric films. These problems include the complexity of the fabrication process and reduced reliability.
Attempts have been made to address these problems. For example, FIG. 1A shows, in cross-section, a “one cylinder stack” (OCS) structure of a capacitor storage node according to the prior art. As can be seen in FIG. 1A, the cup-shaped storage node has a capacitance of about two times larger than that of a simple stack capacitor structure because both outer and inner surfaces of the node can be utilized as an effective capacitor area. FIG. 1B shows, in cross-section, a simple stacked capacitor with an HSG layer on its surface according to the prior art. The simple stacked capacitor with an HSG layer has a capacitance about two times larger than that of a simple stacked capacitor without an HSG layer. One cylinder stack capacitors with HSG layers on both inner and outer surface also can be formed.
FIGS. 2A-2D are cross-sectional diagrams which illustrate a method of fabricating an OCS capacitor with an HSG layer thereon. Referring now to FIG. 2A, a device isolating layer 12 is formed on a predetermined region of a semiconductor substrate 10 to define active and inactive regions. A gate electrode structure 14 is formed over the semiconductor substrate 10. A gate oxide layer also is disposed between the gate electrode structure 14 and the substrate 10. Source/drain regions 16 are formed in the active region adjacent to the gate electrode layer. An interlayer insulating layer 18 is formed over the semiconductor substrate 10 and the gate electrode structure 14. A contact hole 19 is opened in the interlayer insulating layer 18 to expose one of the source/drain regions 16. A polysilicon layer 20 is used as a storage node. This layer is deposited in the contact hole 19 and over the insulating layer 18. A photoresist layer pattern 22 is formed over the polysilicon layer 20 to define a storage node region. A low temperature oxide layer 24 is deposited over the polysilicon layer 20 (including the photoresist pattern 22) to a thickness of about 2,500 Å.
Referring to FIG. 2B, the low temperature oxide layer 24 is then dry etched to form sidewall spacers 24a on the lateral edges of the photoresist pattern 22. Using the photoresist pattern 22 and the sidewall spacers 24a as a mask, a timed etching step is performed on the insulating layer 20 to remove more than half of the original thickness thereof.
The formation of the storage node structure is next addressed and illustrated in FIGS. 2C-2D. After removing the photoresist pattern 22, the polysilicon layer 20 is etched back, using the sidewall spacers 24a as a mask, to form the storage node structure 20a, as shown in FIG. 2D. Subsequently, an HSG layer (not shown) is formed on the surfaces of the storage node 20a. A dielectric film and top plate are then formed on the storage node 20a using conventional techniques.
The above-described method has some drawbacks. For example, the timed etch conducted on the insulating layer may not provide process reliability, and the polymer resulting from the etch back may contaminate the storage node which affects the dielectric characteristics. The etch back using the sidewall spacers as a mask also may cause a variation in storage node thickness. Moreover, since the thickness of the top portion of the storage node is less than 1,000 Å, the storage node may fall down during a cleaning process and the HSG formation thereon may totally consume the storage node and cause it to break off.